The present invention relates to apparatus and machines for bonding elements of a workpiece to one another by application of electrical current.
In the process commonly referred to as "fusing", electrodes are applied to the elements of an electrically conductive workpiece, and electrical current is directed through the electrodes so as to form a bond between the workpiece elements. One electrode referred to as to the "fusing" electrode engages an element of the workpiece adjacent the juncture between elements, and substantial forces are applied through this electrode so as to force the workpiece elements together. The opposite electrode, normally referred to as the "ground electrode" engages one or another of the workpiece elements remote from the juncture. The fusing electrode typically has a relatively high resistivity. An electrical potential is applied across the electrodes causing current to flow through the electrodes and the workpiece. Substantial heat is generated in the resistive fusing electrode so that heat is transferred from the fusing electrode to the engaged workpiece element. The workpiece elements fuse with one another under the combined influences of heat transferred from the fusing electrode and pressure applied by the fusing electrode. By contrast, in the bonding process referred to as spot welding, the major portion of the heat evolved during the process is generated by the electrical resistance within the workpiece itself. Both processes, however, involve application of electrical current by means of electrodes engaged with the workpiece.
Fusing has been widely employed in industry for joining metallic workpiece elementssuch as elements of electrical apparatus. For example, in the manufacture of dynamoelectric machines such as motors and generators, the wires constituting the windings of a rotor may be bonded to the commutator by fusing.
Fusing machines of the type disclosed in U.S. Pat. Nos. 4,510,370 and 4,371,772 can employ a rodlike metallic fusing electrode having a pointed contact region or tip. This rodlike electrode is retained in a bore in a conductive, metallic electrode holder block. The electrode holder block is bolted to a movable ram on the fusing machine. A linkage actuated by a fluid power cylinder is arranged to advance the ram and hence the holder block and electrode towards a workpiece so as to engage the tip of the fusing electrode with the workpiece at a predetermined force for a predetermined time. During this time, electrical current is passed through the electrode holder block to the fusing electrode and the workpiece. A separate ground electrode holder provides for engagement of a ground electrode with the workpiece.
Substantial forces and electrical currents must be transmitted through the fusing electrode in this arrangement. Also, the location of the tip of the fusing electrode with respect to the fusing machine head has a significant influence on the results achieved. To provide for location of the fusing electrode, and to prevent unwanted movement of the fusing electrode with respect to the head, the holder block has features which engage features of the fusing machine ram so as to precisely locate the holder block with respect to the ram. Also, the holder block is provided with an electrode adjusting screw coaxial with the electrode receiving bore. The adjusting screw extends into this bore, and the end of the rodlike fusing electrode remote from the tip bears on this screw. Thus, the position of the electrode relative to the holder block can be adjusted by means of the adjustment screw. Typically, a setscrew is also provided so as to lock the electrode in position after adjustment.
As the tip of the fusing electrode is engaged with successive workpieces and used in repeated fusing operations, the tip gradually wears and erodes. This alters the configuration of the contact region at the tip of the electrode and also changes the location of the contact region or tip relative to the fusing machine ram. Wear and erosion of the fusing electrode contact region, if left uncorrected, can impair the fusing process. Wear and erosion of the ground electrode contact region, although generally less severe, can have similar effects. Accordingly, it has been the practice heretofore to change the electrodes as needed to provide new contact regions. Typically, the fusing electrode has been changed by removing the holder block from the fusing machine head, removing the used electrode and substituting a new electrode. As the new electrode may have dimensions different from the previously used electrode, it has been necessary to readjust the adjustment screw in the fusing electrode holder block to bring the contact region at the tip of the electrode into the desired location with respect to the holder block. Ordinarily, the entire fusing machine is out of service during the time required for these steps. The loss of productive time and interruption in production flow associated with these steps can be significant, especially where the fusing machine forms part of a larger automated production line. Thus, the entire line may be brought to a halt during the time required for electrode changing and adjustment.
One solution to these problems is set forth in copending, commonly assigned U.S. patent application No. 908,566, filed Sept. 18, 1986, the disclosure of which is hereby incorporated by reference herein. In apparatus according to one embodiment disclosed in said '566 application, a turret is used instead of the electrode holder block. The turret has a shaft and a plurality of bores extending radially with respect to the shaft. The shaft and turret are rotatably mounted on a fixed element or frame which in turn is attached to the head of the fusing machine. Each bore is provided with an adjustment screw similar to the adjustment screw used in the ordinary holder block. Before the machine is started, a plurality of rodlike fusing electrodes are engaged in the turret, and adjusted by means of the adjusting screws so that the contact regions or tips of all of these electrodes lie at the same radial distance from the axis of the shaft. During normal operation of the fusing machine, the shaft and turret are locked against rotation so as to hold one fusing electrode and hence one fusing contact region in an operative or workpiece engaging disposition. That particular fusing electrode is engaged with workpieces processed by the machine. Mechanical means are provided for unlocking the turret, rotating the turret through a predetermined angle so as to swing a new fusing electrode into operative disposition and then relocking the turret. These operations can be accomplished quickly, without appreciable loss of productive time or interruption in production flow. To provide electrical power to the turret and hence to the fusing electrode, one or more brushes are movably mounted on the frame and spring-biased into engagement with the shaft so as to maintain electrical contact despite rotation of the shaft. These brushes are similar in principle to the spring-loaded brushes employed with the commutator of an electric motor or the like. The machine disclosed in the '566 application also has means for renewing the ground contact region, as by moving an elongated, bladelike ground electrode so as to present a new region of the ground electrode in operative or workpiece engaging disposition. U.S. Pat. No. 3,313,909 discloses a spot welding machine which likewise employs a turret to hold a plurality of electrodes and means for intermittently turning the turret so as to present a new electrode in operative disposition. The '509 patent likewise employs biased brushes engaging the shaft to carry the necessary electrical current to the turret and electrodes.
Although electrode changers as disclosed in the '566 application provide very substantial improvements, there has been need for still further improvements. Thus, the electrode changer disclosed in the '566 application can provide continuous, substantially uninterrupted productivity only until all of the electrodes mounted on the turret are used and worn to the maximum permissible extent. At that time, the machine must be stopped. All of the electrodes mounted on the turret must be changed, and the position of each new electrode relative to the turret must be adjusted. The fusing machine must be idle during these steps.
Also, the brushes and shaft tend to wear and deteriorate with use. These components must carry substantial electrical currents during the fusing operation. The contact resistance at the mating surfaces of the brush and shaft must be low. To provide this low resistance, the brush and shaft are biased against one another with substantial force. Also, at least the mating surfaces of the brush and shaft must be formed from relatively soft, highly conductive materials, such as copper, silver or alloys of these metals. These factors tend to accelerate wear on the mating surfaces of the brush and shaft.
Thus, there have been substantial unmet needs for still further improvements.